A growing interest in prevention of cardiovascular diseases has focused attention on possible precursors of hypertension in children. Crucial methodological problems, however, have limited ability to move forward in this field. In particular, although the relation of salt intake to the development of hypertension has been recognized as a major theoretical question for a number of years, very little data exist relating habitual salt use and blood pressure in infants, children and teenagers. The primary objective of this study is to develop practical and reliable methods for the collection of such data. Consecutive 24-hour urine specimens, divided into two periods, will be collected daily for a week on approximately 100 children ages 10-12. The variability of the individual must be determined before it is possible to classify an individual relative to the rest of the group. The purpose of this study is threefold: 1) to estimate the variance of sodium excretion within individuals, and 2) to gain practical experience while developing an effective method for collecting 24-hour urines from school children, and 3) to assess whether multiple overnight urine specimens (rather than 24-hour specimens) are adequate to classify children's Na intake. Correlations between urinary electrolytes (Na, K) and blood pressure, and the possible usefulness of diet records and salt preference testing, are secondary aims of this study.